Add support for tablet dials

This adds support for the new libinput tablet dial events introduced in 1.26, which emit relative scroll events. This adds the relevant API for them which we can build upon in the future like allowing them to be rebound in the RebindButtonsFilter plugin.

Note that this isn't part of the tablet Wayland protocol yet, but is in the process of being reviewed still. So no applications will be able to pick up on these emits, they're currently internal to KWin.

wayland-protocols MR: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/298